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RECENT VOLCANISM 

AT AMBOY CRATER 

SAN BERNARDINO COUNTY 

CALIFORNIA 



By RONALD B. PARKER, Assistant Professor of Geology 
The University of Wyoming, Laramie, Wyoming 




Report 76 

California Division of Mines and Geology 
Ferry Building, San Francisco, 1963 




STATE OF CALIFORNIA 






EDMUND G. BROWN, Governor ; 

THE RESOURCES AGENCY 

HUGO FISHER, Adminhtrafor 



DEPARTMENT OF CONSERVATION 






*> 



DeWITT NELSON, Director 



DIVISION OF MINES AND GEOLOGY 

IAN CAMPBELL, State Geologist 



SPECIAL REPORT 76 

Price $1.00 




CONTENTS 

Page 

5 Abstract 

7 Introduction 

7 Geologic environment 

8 Physiography 
10 Amboy Crater 
10 The flows 

13 Collapse depressions and flow units 

19 Pressure ridges 

19 Basaltic blisters 

19 The plateau 

19 Age of the volcanism 
21 Petrography 

23 References 

In 

pocket Plate 1. Geologic map of the Amboy Crater area 

7 Figure 1. Map showing location of Amboy Crater area 

8 Figure 2. Map showing faults and centers of recent volcanism 

in the eastern Mojave Desert 

10 Figure 3. Block diagram of Amboy Crater 

13 Figure 4. Cross-section of Amboy Crater and vicinity 

13 Figure 5. Sketch of flow units emergent from large flow 

13 Figure 6. Map of plateau area showing location of jumbles 

and depressions 

6 Frontispiece. Aerial photograph of Amboy Crater 

9 Photo 1. Bombs from area northeast of Amboy Crater 
9 Photo 2. Eastern side of Amboy Crater 

1 1 Photo 3. Hummocky flows east of cone 

1 1 Photo 4. Ropy surface of pahoehoe type flow 

12 Photo 5. Jointed vesicular surface of a flow 
12 Photo 6. Blocky snout of a flow 

14 Photo 7. Edges of flows near west margin of lava field 

15 Photo 8. Breached pressure ridge 

16 Photo 9. Collapse depression 

17 Photo 10. Large basalt blister 

18 Photo 11. A typical jumble 

20 Photo 12. The largest depression on the plateau 



[3] 



ABSTRACT 

Amboy Crater, a recent cinder cone in central San Bernardino County, is typi- 
cal of the many cinder cones which dot the desert regions of southern California. 
Amboy Crater and the surrounding 24 square miles of lava flows rest on the playa 
of Bristol Lake. The Bristol Lake basin is surrounded by fault block mountains 
composed of Paleozoic metamorphic rocks and granitic rocks of unknown age. 
The rocks of Amboy Crater are olivine basalts. 

Amboy Crater proper is a coalescing group of cones representing at least six 
distinct periods of eruption. Flows surrounding the cones are hummocky and 
disrupted. Most of the lava is pahoehoe, but some block lava is present. The 
abundant collapsed lava crusts are attributed to removal of lava from beneath a 
hardened skin. Upward folds in the lava are in the form of low ridges, oval in 
plan, and have resulted from the collapse of nearby lava crusts and compressional 
folding. Domical basalt blisters are thought to have been formed by laccolithic 
intrusions between layers of already congealed flow rocks. 

A plateau is marked by conical heaps of basalt blocks, and' bowl-shaped de- 
pressions 25 to 300 feet in diameter. These features are probably a product of 
explosions from volcanic sources or of steam pressure generated by lava flowing 
onto water-saturated sediments. 

The volcanic activity is estimated to have taken place less than 6,000 years ago. 






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RECENT VOLCANISM AT AMBOY CRATER, 
SAN BERNARDINO COUNTY, CALIFORNIA 



by Ronald B. Parker 



INTRODUCTION 



Amboy Crater, a complex basaltic cinder cone in cen- 
tral San Bernardino County, California, is 75 miles east 
of Barstow on Highway 66, and 3 miles west of the town 
of Amboy. The cone is surrounded by approximately 24 
square miles of lava flows. Field studies were carried out 
in 1953 and 1956. Mapping was done using aerial photo- 
graphs and U. S. Geological Survey topographic maps 
as a base. 

The writer is indebted to Mr. Yee and Mr. Crowl of 
Amboy for their help in transportation in the field. 
Thanks are extended to Professors Howel Williams and 
G. H. Curtis of the University of California, Berkeley 
for their help and encouragement. Some of the field ex- 
penses were defrayed through the graduate research fund 
of the University of California. Chemical analyses were 
paid for by the University of California. 

Geologic Environment 

The Amboy Crater area is one of a number of centers 
of recent volcanic activity which lie on broad alluviated 
valleys and playas in the central part of San Bernardino 
County. Pisgah Crater and Sunshine Cone near Hector, 
some 40 miles .west of Amboy, are described by Gardner 
(1940, pp. 286-288). 

Eight miles west of Amboy Crater is a small cinder 
cone, Siberia Crater, which is not associated with visible 
lava flows. It is approximately 100 feet high and is sur- 
rounded and partly buried by recent alluvium. Siberia 
Crater is deeply dissected by erosion, and now consists of 
a semicircular ridge concave toward the north. The de- 
gree of denudation enables the observer to see that the 
cone is made wholly of ropy, almond-, and ribbon-shaped 
basaltic bombs which are filled with abundant inclusions 
of foreign rocks which include dunite, lherzolite (olivine, 
enstatite, diallage, spinel), and granitic rocks (see also 
Brady and Webb, 1943, pp. 405-406). Another basaltic 
cone north of Siberia— Dish Hill— is of composite type, 



Frontispiece (opposite page). Amboy Crater from the air— view east of 
north. The bomb field is northeast of the cone (upper right). Photo 
courtesy of Spence Air Photos. 



and is made up of alternating layers of bombs and cinders, 
and flows. It is about 300 feet high and 1,000 feet in basal 
diameter. Both Pisgah Crater and Dish Hill have served 
as sources of commercial volcanic cinders. 

Areas in the Lava Hills and Bristol Mountains to the 
north and west of Amboy Crater are covered by acid 
volcanic rocks including perlite, rhyolite, tuffs and tuffa- 
ceous sediments. These rocks are of late Tertiary age 
(Chesterman, 1957, pp. 443-444). Gardner (1940, p. 289) 
says that the rhyolitic flows in the Newberry Mountains 
are Pleistocene, and suggests correlation with the Bristol 
Mountain series. The sources of the Bristol Mountain 
acid volcanic rocks are in close proximity to the bodies 
themselves (C. W. Chesterman, personal communica- 
tion). 

Pre-Tertiary rocks in the Bristol Mountains are pri- 
marily plutonic igneous rocks, and Paleozoic sedimentary 
and metomorphic rocks. Porphyritic granite and quartz 
monzonite are common. Dioritic types are widespread, 
but are not as abundant as the more felsic varieties. Quartz- 
muscovite-feldspar pegmatite and aplite are commonly 
associated with the felsic rocks. Dolomite and contact 
metamorphic rocks (at dolomite-granite contacts) are 

FIGURE 1. Map showing the location of the Amboy Crater area. 





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California Division of Mines and Geology 



[Special Report 76 



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FIGURE 2. Map showing the location of centers of recent volcanism, and their relationship to faults in the eastern Mojave Desert. The 
distribution of the volcanic centers does not appear to be related to the fault pattern. 



present. Marble and pelitic schist are common in the Mar- 
ble (Iron) Mountains to the east. Mining activity is con- 
fined at the present time to prospects for a variety of 
metallic ores, and the perlite deposits mentioned above. 
The extensive saline and gypsum deposits of Bristol Lake 
are described by Gale (1951) and Ver Planck (1952, p. 
47). 

Physiography 

The Amboy Crater area is within the physiographic 
province termed the basin and range province, the main 
features of which are generally attributed to faulting. 
The ranges are thought to be block mountains bounded 
by faults along which the inter-range basins have sub- 
sided with respect to the elevated mountain blocks. This 
simplified picture is modified by erosion, the filling of 



basins with alluvium, and some thrust faulting. The dis- 
tribution of centers of recent volcanism, and of faults in 
part of this province, is shown in figure 2. A site of vol- 
canic activity is commonly supposed to coincide with a 
weak place in the crust, such as a fault or the intersection 
of faults. Most of the centers shown in figure 2 do not 
bear out this simple relationship— but many more faults 
than are shown doubtless exist. 

The Bristol Lake basin is probably a graben (a fault 
trough), but its shape has been modified by alluvial fan 
and outwash plain deposits from the mountains to the 
north and south. The drainage from these highlands is 
now confined to the basin itself, no outlet to the sea being 
in evidence. Darton (1916, p. 153) and Blackwelder 
(1954, p. 36) proposed that Bristol Lake is part of a Pleis- 



1963 



Am boy Crater 




Photo 1 (left). Typical almond-shaped bombs and ribbon bombs from 
the area northeast of the cone. The smooth shapes are indicative of 
ejection in a fluid state. Cooling and hardening of the bombs took place 
in the air, prior to their falling to the ground. 



Photo 2 (below). Eastern side of Amboy Crater, showing gullies in the 
old cone surface. The gullies are filled with later basaltic material near 
the rim of the crater and on either side of the photograph. The irregular 
flow surface in the foreground is typical. 







10 



California Division of Mines and Geology 



[Special Report 76 




FIGURE 3. Block diagram of Amboy Crater. Stages in the development of the complex cone are 
numbered from oldest to youngest. 



tocene river system which drained from Death Valley to 
the Colorado River. In any case the Bristol Lake basin 
has been in existence in much its present form for a long 
enough time for the salines to accumulate. 

AMBOY CRATER 

Amboy Crater rises approximately 250 feet above the 
surrounding lava flows, and is about 1,500 feet in basal 
diameter. It is composed of a loose accumulation of vol- 
canic ejecta (material thrown out by volcanic explosions) 
with secondary amounts of agglutinated ejecta and flows. 
The ejecta range widely in type, angular scoriaceous cin- 
ders predominating over ropy, ribbon- and almond- 
shaped bombs. The angular cinders were doubtless 
thrown out in a semi-solid state, while the other types 
were somewhat more fluid and acquired their stream- 
lined shapes through being chilled while spinning or 
twisting in the air before falling to the surface. Some 
lithic nonvesicular accessory basaltic ejecta are present, 
but included foreign or accidental fragments are absent. 

Amboy Crater is not a single cone, but is complex; it 
may be thought of as a group of at least four nearly 
coaxial nested cones. The outer slopes of the main cone 
are gullied by erosion and avalanches of loose debris ex- 
cept near the crater rim and on the western flank; there 
the gullies have been obliterated by an agglutinated mass 
of basaltic blocks erupted after the gullies formed. Within 
the main outer cone there is a remnant of a second cone 
on the west side. Both of these cones are breached on the 
west side. The breach is now occupied by a short lava 
tongue. In addition to the two cones, there are two rela- 
tively undisturbed cone walls within the main crater. 
These innermost conelets are composed almost entirely 
of angular scoriaceous cinders; ropy bombs are absent. 

From the above data the following sequence of events 
at the main vent of Amboy Crater has been synthesized. 

1. Early eruptions were explosive, and many fluid bombs were 
erupted. The main cone was formed at this time. A period of in- 
activity followed, during which the outer slopes of the cone were 
gullied by erosion or by avalanching volcanic material. 

2. A second phase of eruptive activity resulted in the deposition 
of an agglutinated aggregate of basaltic blocks on the rim and 



western flank of the cone. This was probably accomplished by 
the eruption of pasty bombs. 

3. The outermost inner conelet was formed by a mild explosive 
phase. 

4. The two cone walls were then breached on the western side 
by a sideways-directed explosion or by the flow which now occu- 
pies this breach. 

5. Activity was renewed with the formation of another inner 
conelet. 

6. Formation of the innermost conelet terminated the explosive 
eruptions at the volcanic center now occupied by Amboy Crater. 

7. Subsequent eruptions, if any, from this central vent took the 
form of quiet outpourings of fluid lava from the base of the main 
outer cone. 

The above relations are shown schematically in fig- 
ure 3. 

Thus a minimum of six distinct periods of eruption 
can be induced from the field evidence at Amboy Crater. 
Petrographic evidence which supports this supposed se- 
quence of events is discussed in another paper (Parker, 
1959). Basaltic fragments which could be identified posi- 
tively as ejecta are found only within the area designated 
limit of bo?nbs on plate 1. Bombs are not found on the 
flows to the south and west of the cone. This relationship 
leads to the conclusion that explosive activity had ceased 
or greatly diminished before the last outpourings of lava 
as flows, since one would expect to find some bombs in 
these areas if they had not been buried by subsequent 
flows. This is in accord with observations at the Mexican 
cinder cone Paricutin where early explosive eruptions 
gave way to quiet effusive outpourings of lava (Howel 
Williams, personal communication). 

THE FLOWS 

The flows are hummocky and irregular and contain 
numerous arched portions, collapse depressions, blisters, 
irregular tongues, and blocky slopes. The irregularity of 
the surface and a discontinuous cover of sand make an 
evaluation of the location of the vent or vents from 
which this lava poured uncertain. Flows total a greater 
thickness in the area of the cone and of the plateau 2 
miles southeast of the cone. Individual flows range from 
a foot to 12 feet in thickness. There are many individual 



1963 



Am boy Crater 



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Photo 3 (above). Hummocky flows east of the cone area. The irregularity here is due to closely spaced 
wrinkles caused by collapse of portions of the lava surface. In the foreground is a desert pavement of 
small basalt fragments which lie upon windblown sand. 



Photo 4 (below). Ropy surface of a pahoehoe flow. The wrinkles were caused by folding of the viscous 
skin as more liquid lava flowed beneath it. The original glassy surface has been removed by sandblast action. 







12 



California Division of Mines and Geology 



[Special Report 76 




Photo 5. Jointed surface of a flat-lying flow, show- 
ing the scoriaceous or vesicular character. Drifted sand 
fills joint cracks. Width of the area covered by the 
photo is above 4 feet. 



Photo 6. Blocky snout of a thick flow west of the cone. Earlier flows, to the left of the blocky portion, 
are partially covered by sand. 




1963 



Amboy Crater 



13 



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FIGURE 4. Cross-section of Amboy Crater and vicinity. The cross-section is schematic, with a greatly 
exaggerated vertical scale. 



flows which must have issued from several vents in this 
area. Two locations which are particularly likely vent 
sites are the area of the cone and of the plateau. A sche- 
matic cross-section of Amboy Crater area is shown in 
figure 4. 

Many of the flows are of the pahoehoe type, and ex- 
hibit typical ropy, twisted, and wrinkled surfaces. Blocky 
material is not uncommon, however, especially at the 
snouts of some of the flows. It was not possible to trace 
the areal extent of individual flows because of exposure 
problems and lack of obvious lithological distinctions. 

Collapse Depressions and Flow Units 

Horizontal flows in the area are scarred by numerous 
oval or circular depressions. These depressions are from 
a few feet to 150 feet in mean diameter, and are of un- 
certain depth, as most of them are partially filled with 
sand; but depths of one-quarter the diameter are not un- 
common. The depressions, which are commonly bounded 
by collapse fractures, are inferred to be the result of 



FIGURE 5. Sketch of flow units emergent from parent flow. Parent 
flow on left is pock-marked with collapse depressions. Flow units on right 
side have themselves been parent flows to additional flow units. Area 
covered by sketch is approximately 600 feet in width, (from an aerial 
photograph.) 



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gravitative collapse of a cooled crust subsequent to with- 
drawal of molten lava from beneath. The lava which was 
withdrawn formed smaller secondary flows or flow units 
(Nichols, 1936, pp. 617-624) in front of the parent flow. 
This sequence may recur several times in the course of 
the advance of a single flow, giving, a gross stepped ap- 
pearance to some flows (fig. 5). Undisturbed lava tunnels 
were not found, but collapsed remnants are widespread. 



FIGURE 6. Map of the plateau area, enlarged from a portion of 
plate 1. Pits and jumbles lie along nearly straight lines. This area may 
have been underlain by volcanic vents. 




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14 



California Division of Mines and Geology 



[Special Report 76 




Photo 7. Edges of a number of thin flows near the western margin of the lava field. The thin 
flows have a large horizontal extent. 



1963] 



Amboy Crater 



15 




Photo 8. A breached pressure ridge. This arch was formed by the collapse of flows to either 
side. The figure of the man indicates the size of the ridge. 



16 



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Amboy Crater 



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Pressure Ridges 

Pressure ridges (Nichols, 1946, pp. 1049-1086) or 
breached basaltic barriers (Skeats and James, 1937, pp. 
245-291) are common in the Amboy Crater flows. They 
are elongate domical folds in the flows which range from 
low wrinkles to large folds 150 feet v long and 50 feet 
high. They are oval in plan, and nearly symmetrical 
along a vertical axial plane; most of them are breached 
by a lengthwise trough from 4 to 20 feet in depth and 
from 2 to 10 feet in width. The walls of these breaches 
are strongly grooved, suggesting that adjacent blocks 
scraped together during folding. 

The areal distribution of the ridges is random. As may 
be seen by an inspection of plate 1, the axes of the pres- 
sure ridges bear little or no relationship to the direction 
of flow of the lava. They appear to have formed pri- 
marily as a result of the collapse of nearby portions of 
the lava crusts. Had the pressure ridges formed during 
the advance of a moving flow, one would expect that 
they would be asymmetrical with the steep side facing 
the direction of flow. A discussion of the origin of com- 
parable features in New Mexico is presented by Nichols 
(1946, pp. 1049-1086). 

Basaltic Blisters 

Several dome-shaped folds of lava, circular in plan, are 
present in addition to the pressure ridges (photo 10). 
They are about 100 feet in diameter and as much as 30 
feet in height. They are characterized by a summit de- 
pression 3 to 4 feet across, and one or more radial frac- 
tures leading outward from this depression. The doming 
seems to be a result of internal pressure, inasmuch as the 
surface fractures are tensional. Some of the domes are 
disrupted as if by weak explosions. Jagger (1931, pp. 
1-3) has attributed the formation of similar features in 
Hawaii to the intrusion of a laccolithic (lens-shaped) 
mass of lava between layers of already cooled flows. The 
final stage of the process is the squeezing out of fluid lava 
or the building of a summit spatter cone on the blister. 
These secondary features were not observed at Amboy 
Crater, but the general principles of the mechanism seem 
applicable. 

THE PLATEAU 

The area designated on the map (plate 1) as the plateau, 
and shown enlarged on figure 6, is of interest because of 
several features which are peculiar to the plateau area. 
These features are of two types. 

First, there are 14 piles of chaotically arranged blocks 
of basalt (photo 11). The surfaces of many of these 
blocks are red, presumably indicating the presence of 
ferric iron in some form, a feature not observed else- 
where in the area except on the surfaces of a small num- 
ber of bombs. Olivine crystals in some of the basalt from 
the jumbles are partly replaced by red-brown alternation 
products. The jumbles range in size from 8 to 40 feet in 
diameter and from 3 to 12 feet in height. 



Second, there are 12 bowl-shaped depressions, some 
with raised rims of basalt blocks. These depressions are 
confined to the southern half of the plateau while the 
jumbles are confined to the northern half. The depres- 
sions are from 25 to 300 feet in diameter, and from 4 to 
40 feet in depth. At two places the central depression is 
surrounded by a roughly concentric polygonal fracture 
along which the central area has subsided, forming basins 
within basins. The larger of the two is now a depression 
of 900 feet overall diameter (see photo 12). 

It seems likely that both the jumbles and the depres- 
sions were produced by explosions. It is difficult to ascer- 
tain the source of the explosive force. An examination 
of figure 6 will show the tendency for the jumbles and 
depressions to lie along nearly straight lines. It could be 
concluded from this relationship that these features are 
concentrated along and above steep planes of weakness 
or topographic highs in the subsurface rocks. The red- 
dening of the outer surfaces of the blocks and the for- 
mation of alteration products were probably due to the 
action of steam on heated rocks. The reddening probably 
resulted from the formation of ferric oxides and hy- 
droxides from iron-rich basaltic glass and magnetite. 

There are two possible explanations which satisfy the 
above evidence. First, the area could represent outpour- 
ings of lavas from a secondary vent from which the 
eruptions were primarily effusive— except for minor ex- 
plosive activity. A second possibility is that lava from 
another source flowed onto wet sediments on the floor 
of the playa, and that the resulting steam was trapped 
beneath the advancing flow, causing phreatomagmatic 
(Stearns, 1953, pp. 599-600) explosions similar to those 
in Iceland described by Thorarinsson (1953, pp. 30-41). 

Abundant steam would be present in either case. A 
feature favoring the phreatomagmatic hypothesis is the 
complete absence of recognizable ejecta, which would be 
expected had there been vents in the area. On the other 
hand, the fact that the flows total a greater thickness 
under the plateau suggests the presence of one or more 
secondary vents. 

AGE OF THE VOLCANISM 

Rocks at Amboy Crater as well as those of the flows 
have been but slightly attacked by weathering. Cinders 
of the cone itself are not visibly affected; those on the 
surface appear the same as those one finds by digging a 
few feet below the surface. The only effects of erosion 
present are the gullies on the outer wall of the cone, and 
these are so slight as not to appear on the topographic 
map at a scale of 1:24,000. 

Fluting and polishing by wind-driven sand is common 
on the flows, but is confined to a few inches above the 
surface and to those places where the amount and veloc- 
ity of wind is intensified by circumstances of topography. 
In most places angular details of the flows are very per- 
fectly preserved, and tachviitic (glassy) selvages are 
common. 



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[Special Report 76 



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Sediments of Bristol Lake have— to a very limited 
extent— lapped onto the eastern and southern margins of 
the flows. Borings near the contact between the lava and 
the lake beds disclosed no buried flows. On the north- 
western lava-sediment contact, flows are partially covered 
by sandy alluvium of the outwash plain from the Bristol 
Mountains; but deposits of this sort accumulate rapidly. 

The volcanic features in general imply very recent 
activity; Amboy Crater appears to be younger than many 
cinder cones and flows along the eastern front of the 
Sierra Nevada in Mono County which are described by 
Blackwelder (1931, p. 891) as interglacial. Probably the 
activity at Amboy Crater was post-glacial, for one would 
expect to find flows more extensively buried by lake 
beds and alluvium if they had been present during a moist 
glacial cycle. 

While no definite age may be assigned from this limited 
and uncertain evidence, it does seem likely that the vol- 
canism was post-Tioga (the latest glacial stage in Cali- 
fornia). The end of Tioga glaciation has been estimated 
by Mumford (1954, p. 18), on the basis of C 14 dates, as 
approximately 6,000 years ago. 



PETROGRAPHY 

All of the rocks of volcanic origin at Amboy Crater 
are fine-grained, porphyritic olivine basalt, for the most 
part vesicular. Olivine (Fa 5 to Fa 23 ) and a minor amount 
of plagioclase (An 45 . 60 ) form the phenocrysts. The 
groundmass is composed of brown basaltic glass (refrac- 
tive index = 1.58-1.60), plagioclase microlites (An 47 . 53 ), 



clinopyroxene (pigeonite ?), and magnetite. Calcite and 
zeolites (?) are present as amygdules in a few specimens. 
The chemical analysis given below is an average of three 
analyses of rocks from Amboy Crater. 

A suite of rocks from the crater itself was examined 
with a view to determining chemical and mineralogical 
variations, if any, with decreasing age of eruption as 
established from field evidence. Regular changes in min- 
eralogy and chemical composition verify the proposed 
sequence of eruptions. The details of petrography of 
these rocks are discussed elsewhere (Parker, 1959). 

Average * chemical analysis of basalt from Amboy Crater. 



SiO* 
TiO* 

AbOa 
FeaOa 

FeO 

MnO 

MgO 

CaO 

Na^O 

K2O 

H^O-f- 

H2O- 

P2O5 

co 2 



Weight percent 

46.97 

2.32 

16.71 

4.94 

5.35 

.19 
7.24 
9.10 
3.14 
1.66 

.65 

.39 

.28 
1.06 



100.00 



* Average of three specimens from Amboy Crater. W. H. Herdsman, analyst. 






REFERENCES 

Blackwelder, Eliot, 1931, Pleistocene glaciation in the 
Sierra Nevada and Basin Ranges: Geol. Soc. America 
Bull., v. 42, p. 865-922. 

Blackwelder, Eliot, 1954, Pleistocene lakes and drainage 
in the Mojave region, southern California: California 
Div. Mines Bull. 170, chap. V, part 5, p. 35-44. 

Brady, L. F., and Webb, R. W., 1943, Cored bombs from 
Arizona and California volcanic cones: Jour. Geologv, 
v. 51, p. 398-410. 

Chesterman, C. W., 1957, Pumice, pumicite, perlite and 
volcanic cinders: California Div. Mines Bull. 176, p 
433-448. 

Darton, N. H., and others, 1916, Guidebook of the west- 
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Survey Bull. 613. 

Gale, H. S., 1951, Geology of the saline deposits, Bristol 
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fornia Div. Mines Special Rept. 13. 

Gardner, D. L., 1940, Geology of the Newberry and Ord 
Mountains, San Bernardino County, California: Cali- 
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Hewett, D. F., A fault map of the Mojave Desert region: 
California Div. Mines Bull. 170, chap. IV, part 1. 

Jagger, T. A., 1931, Lava stalactites, stalagmites, toes, and 
"squeeze-ups": The Volcano Letter, 345, p. 1-3. 

Mumford, R. W., 1954, Deposits of saline materials in 
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chap. VIII, part 2, p. 15-22. 

ichols, R. L., 1936, Flow units in basalt: Jour. Geology, 
v. 44, p. 617-630. 

Nichols, R. L., 1946, McCarty's basalt flow, Valencia 
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p. 1049-1086. 

Parker, R. B., 1959, Magmatic differentiation at Amboy 
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Skeats, E. W., and James, A. V. G., 1937, Basaltic bar- 
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Thorarinsson, Sigurdur, 1953, The crater groups in Ice- 
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Ver Planck, W. E., 1952, Gypsum in California: Cali- 
fornia Div. Mines Bull. 163. 





A82498 3-63 3,500 



printed in California state printing office 



STATE OF CALIFORNIA 

THE RESOURCES AGENCY 

DEPARTMENT OP C 




EXPLANATION 



Flow direction 



Pressure ndqe 



AMBOY CRATER 



LEAD MTN..NE 



Base mop assembled in 
U.S6S 71/2 quadronglt 



d 



MAP OF AMBOY CRATER AREA BASALT FLOWS 
SAN BERNARDINO COUNTY, C ALI FORN I A